Most engine and motor fuel is comprised of hydrocarbons (molecules having the formula CxHy; generally, for linear, non-cyclic alkenes, this formula can be simplified to CnH2n+2. Although the present invention may have application to the cleaning and “polishing” of fuels other than hydrocarbon fuels (such as H2 gas), in many applications the present invention concerns hydrocarbon fuels, particularly though not necessarily exclusively, diesel fuels.
Most engine fuel is produced from petroleum (literally “rock oil”) as a hydrocarbon mixture made from the fractional distillation of crude oil. Diesel fuel is a definitional term encompassing any fuel that can be used in a diesel engine. Generally, petroleum diesel fuel is made using alkanes from C9 (nonane) to C16 (hexadecane), while gasoline is made from alkanes from C6 (hexane) to C8 (octane). Petroleum diesel has a greater energy density than gasoline, and is easier to refine and produce than gasoline. Other types of diesel fuels can be made from organic or other sources, such as biodiesel, biomass to liquid (BTL) or gas to liquid (GTL) diesel.
A requirement of a diesel fuel is that it be ignitable in the absence of a high voltage spark plug. Due to the manner in which a diesel engine functions, the fuel must ignite when the air inside the cylinder is compressed up to 15 or 25 times its volume at atmospheric pressure and the fuel is then injected into the cylinder at the end of the compression stroke.
Contamination of fuels can be costly and dangerous. Water is the greatest concern because it is the most common form of contaminant. Water may be introduced into the fuel supply during fueling when warm, moisture laden air condenses on the cold metal walls of fuel storage tanks, or from poor housekeeping practices, such as leaky valves, hoses and the like. Water contamination of liquid fuels may result in explosions as heated water becomes steam at a relatively low temperature and expands. Alternatively, the water may stall the engine leading to a potentially dangerous power failure.
Water can also cause damage to injector components and reduce the lubricity of the fuel, which can cause seizure of close tolerance components such as those found in fuel pump assemblies.
Microorganisms, such as fungi and bacteria, live in water and feed on the hydrocarbons found in virtually any hydrocarbon fuel. Bacterial and fungal colonies can multiply and spread throughout a fuel system and quickly plug a fuel filter. The fuel filter will have a slime-like coating over the surface of the media, dramatically reducing the service life of the filter. Bacteria may be any color, but are usually black, green or brown. Draining the system will reduce microbial activity, but will not eliminate it.
Wax crystals can form in diesel fuel as a result of cold temperature precipitation of paraffin from the fuel. These crystals begin as microscopic particles suspended in the fuel, but can grow. Temperatures below a fuel's cloud point will result in wax precipitation and filter plugging. To prevent plugged filters due to wax formation, the cloud point of fuel must be at least 12° Celsius (22° F.) below the lowest outside temperature. Fuel suppliers blend diesel fuel based on local anticipated cold weather conditions. Thus, wax formation is particularly problematic if fuel is obtained outside the locality in which it is used.
Asphaltenes (which are also components of crude oil) are by-products of fuel as it oxidizes. Asphaltene particles are generally thought to be in the 0.5-2.0 micron range and are relatively harmless to the injection system, as they are soft and deformable. However, as these tiny particles pass through the filter they tend to stick to the individual fibers. If a choked filter is cut open after a normal service interval a black, tarry substance would be visible on the dirty side of the element; this is asphaltene (oxidized fuel). Fuel with a high percentage of asphaltenes will drastically shorten the life of a fuel filter.
Numerous surveys have been performed monitoring fuel tanks aboard commercial ships such as tankers and freighters, as well as military ships. On one such survey, sludges collected from fuel tanks on DD-963 ships (Spruance-class destroyers) were assessed to determine the sources of particulate matter, especially particulate matter generated or caused by microorganisms. These ships are powered using gas turbines, which are usually fueled with turbine fuel, which is similar to diesel fuel.
More than eighty fuel tanks on eight different ships were sampled and examined microscopically for the presence of microorganisms and other debris. Assays for viable fungi, yeast and bacteria, including sulfate reducers, were made. Centrifugal fractionation separated the fuel and aqueous phases of the sludges and allowed the volumes of low-density particulates (lighter than water) and sediment to be estimated. The pH and salinity of the aqueous phase were measured and analyses made for organic matter, sulfide and metallic elements in the particulate matter. There were considerable variations in amount, quality and microbial content of the sludges in different tanks even on the same ship. Viable microorganisms were always found but the dominant genera differed considerably. A high aqueous pH and the presence of sulfide were usually correlated with active sulfate-reducing bacteria and a low aqueous pH was always associated with high yeast and fungal content. In some tanks, including service tanks, fungal material made up a substantial portion of the sludge.
Sludge deposits are not unique to fuel tanks on military or commercial ships, and can be found in the tanks of pleasure boats as well. Additionally, of course, the formation of sludges and the contamination of fuel tanks and fuel lines is a common feature of any equipment or vehicle in which fuel may sit unconsumed for a significant period of time.
Thus, fuel, particularly hydrocarbon fuel, stored for any length of time, and particularly in tanks subjected to water infiltration, condensation and the like, are susceptible to microbial contamination or chemical degradation. Besides marine applications, this may apply to applications including trucking, heavy machinery (such as cranes, bulldozers, tractors and the like), oil well pumping and drilling, service station and military fuel storage, and the like.
So-called “fuel polishing” is often used to clean and filter fuel stored in tanks that might have become, or which is at risk of becoming, contaminated. This generally involves using a recirculating pump to draw stored fuel from a tank, direct it through one or more filter and/or contamination removal devices, and direct the fuel back into the tank. Generally the fuel is circulated at a high enough rate to agitate the inside surface of the fuel tank, and suspend sludge that may have formed, particularly on the bottom or sides of the tank. In this way, the suspended sludge and particulate matter can be directed to the filters and the inside of the tank may also be cleaned during the fuel polishing operation.
Generally, when the fuel in one tank has been sufficiently cleaned, the pump (including any hosing and filter apparatus attached thereto) is moved to another tank (if present) and the process is repeated until the fuel in all the tanks (and optionally the fuel line(s)) has been filtered free of sludge, algae, bacteria, and non-biological impurities.
For example, in a commonly performed fuel polishing procedure, a boat or ship is brought alongside a fuel dock, and an inlet hose and an outlet hose is introduced into the fuel tank. A pump connected to one or more filter is then turned on and permitted to run at sufficient velocity to cause the suspension of sludge, water, and solids in the fuel, and for sufficient time to substantially filter the fuel in the tank. Occasionally filters may be changed, typically beginning with a large pore filter, and decreasing the pore size until a substantial amount of the fungi and bacteria can be removed from the fuel.
Similarly, small “on-board” or integrated fuel polishing systems are known, which generally involve a pump that circulates fuel through one or more filters, and manually switching valves to change the source, filters or filterbanks, and/or destination fuel tanks.
While fuel polishing systems are presently marketed and sold, these systems consist largely of a series of filters, valves and hoses, which may also contain timers for periodic circulation of the fuel in a single tank. These systems may also contain valves, such as two-way or three-way valves, for manual switching from tank to tank.